Scholars who believe it is real have said it predates Christopher Columbus and proves he was not the first European to reach America, bolstering research into archaeological discoveries in Canada.
Both studies were published independently in scholarly journals, the researchers announced Monday.
One study, done by researchers at University College in London, analyzed the map's ink under a Raman microscope and concluded the map was produced after 1923.
"The results demonstrate the great importance of modern analytical techniques in the study of items in our cultural heritage," said Robin J.H. Clark, a University College professor who did the analysis.
Not so fast, says another team of researchers, which determined through carbon dating that the map's parchment was produced around 1434, exactly the right time for the map to be authentic.
"It's not a trivial thing for a forger to get a parchment" from that time period," said Jacqueline S. Olin, a research chemist recently retired from the Smithsonian Institution.
Olin took part in a study done by the Smithsonian Institution, the University of Arizona and the U.S. Department of Energy's Brookhaven National Laboratory. The study will be published in the July issue of the journal Radiocarbon.
The authenticity of the map has been debated since the 1960s, when benefactor Paul Mellon gave the map to Yale University.
The map depicts the world, including the north Atlantic coast of North America. It includes text written in medieval Latin and a legend that describes how Leif Eiriksson, a Norseman, found the new land called Vinland around the year 1000.
Scholars have dated the map to around 1440, 50 years before Columbus sailed to the New World. Some scholars have speculated that Columbus could have used the map to find America.
The map was included in a book called the Tartar Relation, a medieval travelogue. The map was sold in the 1950s by a dealer in rare Spanish books to a Connecticut dealer, who then sold it to Mellon. The original dealer died without revealing his source.
The map, which has been valued at more than $20 million, is housed at Yale's Beinecke Rare Book and Manuscript Library.
Yale has not taken a position on whether the map is authentic, said the university's head librarian, Alice Prochaska.
"The truth is there has been a good deal of controversy about the Vinland Map. There is a lot of research in progress," Prochaska said. "I think probably research will reveal one day what the truth is, but it is certainly very much under discussion and debate."
The research by Clark and a colleague, Katherine L. Brown, is published in the July 31 issue of Analytical Chemistry, the journal of the American Chemical Society.
They followed up on research that Chicago chemist Walter C. McCrone Jr. did in the 1970s.
McCrone, who died earlier this month, had peered through microscopes to cast doubt on the genuineness of the Shroud of Turin and to conclude that Beethoven died of lead poisoning.
He focused on the map's ink, which consists of a black layer that is flaking off over a yellowish layer that adheres firmly to the parchment.
McCrone found round, uniform crystals of anatase in ink. Anatase, a form of titanium dioxide, has been used to produce inks since the 1920s.
Anatase is found in nature, but in small amounts that would be found in jagged, irregular crystals if a medieval scribe had used it to make the Vinland Map's inks, he said. Based on this conclusion, McCrone pronounced the map a fake.
Clark's study also focused on the ink. He used a Raman microscope, which uses a laser beam that scatters off molecules as radiation with different colors. Every material has a unique scattering spectrum.
Clark found that anatase was detected solely in the yellowish ink lines, and not elsewhere on the parchment.
Yellow lines are sometimes left behind when medieval ink, made of iron gallotannate, degrades.
But, the black ink on top of the yellow ink was found to be carbon-based, not iron gallotannate, so no yellow residue should be present, Clark said.
Clark said that a forger would know about the yellow residue, so he would try to reproduce it by drawing the map first with yellow ink, then with black ink.
Also, the map has not grown brittle over the years, as would be expected with an iron gallotannate ink, he said.
Olin and other researchers said, however, that iron also has been found in the black ink, so Clark's study does not prove the map is a forgery.
Their study used a thin strip of parchment taken from the map to date it with a mass spectrometer.
The results showed the map dates back to 1434 — about the time that scholars had dated the map, based on the text and comparisons to other old maps.
"The question of whether the parchment came from the period is settled," Olin said.
However, skeptics say that a clever forger would have known enough to get the exact kind of parchment used at the time. And, carbon dating cannot be done on the ink because the sample is so small, they say.
Another expert on the map, Thomas A. Cahill, a professor of atmospheric science and physics at the University of California at Davis, also has debunked Clark's idea of two inks.
It would be impossible for a forger, no matter how clever, to draw the map in titanium-based ink and draw over those lines, perfectly, in black ink, Cahill said.
"The wildly improbable double inking proposition should have died a violent death 100 years ago," said Cahill.
Also, Cahill said, the Latin writing on the map has been deemed authentic, and other parts of the map's story add up.
"We can neither prove or disprove the map is authentic, but I think some claims proving it's a forgery need closer examination," Cahill said. "If it's real, it fits in with a lot of the facts."